Concrete is considered a brittle material because of its low tensile strength and strain and thus requires reinforcement for example steel reinforcement rod such as rebar to provide a structural concrete generally known as reinforced concrete.
Another form or method of reinforcing concrete is to form a composite incorporating short fibers such as steel fibers, which typically have a length of approximately 25 mm (1 inch). By dispersing these fibers throughout the concrete, the fracture toughness of the concrete can be increased several times so that the amount of energy consumed prior to rupture is significantly greater. One form of concrete wherein the fiber reinforcing is especially attractive is concrete known as Shotcrete which is a form of concrete having dispersed therein a plurality of fibers that are sprayed together with the cement, water and aggregate to produce a fiber reinforced Shotcrete when the cement sets in situ. Approximately, 50% of the total worldwide steel fiber demand is consumed by Shotcrete.
One of the major problems with steel fibers used in Shotcrete is known as "rebound" which occurs when the dry-mix Shotcrete mixture of cement aggregate and fiber is sprayed or shot into position in that a high proportion of the fibers fails to become embedded in the resultant concrete and thus, are wasted. For example, with commercially available fibers which generally have a diameter of about 0.5 mm (some flat fibers are also used) and a length of about 25 mm as much as 75% of the steel fiber may rebound and not be present in situ in the final concrete.
It is recognized that reinforcing fibers being pulled out of the concrete matrix at cracks is the main mechanism that allows steel fiber reinforced concrete (SFRC) to be more ductile than unreinforced concrete. Thus, all commercial reinforcing fibers presently available in the market, are deformed at the ends or along their length, to enhance the anchorage of the fiber with the concrete matrix and generate a greater pullout resistance.
The state-of-the-art in fiber design may be divided into two large groups with respect to their anchorage mechanisms, namely a "dead anchor" and a "drag anchor".
Dead anchors generally are produced by deforming the fiber with a hook or cone adjacent to each of its ends. Under stress, in an aligned fiber (i.e. under axial tension) the anchor is generally designed to fail (e.g. pullout) at a maximum resistance below the strength of the steel. However, these dead anchors, after failure, have a significantly reduced capacity to resist pullout displacement.
Drag anchors generally are formed by enlarging the fiber adjacent to its end in such a way that during pullout, the enlargement generates friction with the matrix as the fiber is dragged out of the concrete. This type of fiber generally develops a lower maximum pullout resistance as compared to the dead anchor but its effect tends to last for a greater pullout displacement and therefore a greater pullout energy is consumed by the end of the pullout process.
Various types of anchoring mechanism are shown for example in U.S. Pat. No. 4,883,713 issued Nov. 28, 1989 to Destree et al. which shows reinforcing fiber with an expanded head at each axial end of the fiber and U.S. Pat. No. 5,215,830 issued Jun. 1, 1993 to Cinti which shows a metal wire reinforcing fiber with a straight central portion and offset anchoring parts at opposite ends. Canadian patent 2,094,543 published Nov. 9, 1993 inventor Nemegeer which discloses a fiber with hooked ends.
U.S. Pat. No. 5,443,918 issued Aug. 22, 1995 to Banthia et al. discloses a metal fiber for reinforcing cement based material which incorporates sinusoidal shape end portions deformed in a specific manner tailored in accordance with the fiber and matrix properties to obtain the desired composite toughness in the resultant composite.
U.S. Pat. No. 5,451,471 issued Sep. 19, 1995 to Over et al. describes a reinforcement fiber deformed near both of its ends over a selected distance so that a selected amount of the undeformed portion of the fiber is between the deformities. The fibers are also provided with a large number of notches that extend at an angle to the longitudinal axis of the fiber and increase pullout resistance of the fiber when used as reinforcement in the concrete matrix.